The present invention relates to a control method responsive to an abnormal sensor output in a control system whose control operation is performed in accordance with the output of a sensor and more particularly to an abnormal sensor output responsive control method so designed that the operation of an accessory device of an automobile (hereinafter referred to as a vehicle) to be controlled is controlled by monitoring the outputs of a main and an auxiliary sensor and that the device is controlled suitably in accordance with the output of the auxiliary sensor upon detection of the abnormality of the output from the main sensor.
Where the accessory devices of a vehicle such as a vehicle air-conditioner, heater and automatic wiper are automatically controlled with the use of various sensors, if a fault occurs in a part of the control system including the sensor, etc., so that the accessory device is brought into an unexpected operation, this causes a feeling of uneasiness on the part of the user and not desirable to the user. Known wiper control devices are shown in Japanese Laid-Open Patent Applications No. 58-64553, No. 58-178043 and No. 58-179259 and an example of this type of automatic wiper control device is designed so that the light output of an infrared light emitting element (hereinafter referred to as an infrared LED) is received by a light sensor (main sensor) including photodiodes through the windshield whereby when rain drops deposit on the windshield, a decrease in the light sensor output is detected to bring the wiper into operation in rainy weather and the wiper operation is placed in an OFF mode, intermittent mode, low speed (low) mode or high speed (high) mode of operation in accordance with the rate of decrease of the output from the light sensor. This type of control device is disadvantageous in that if the infrared LED or the photodiodes become faulty or the optical axis is displaced by vibrations or the like or the light output from the infrared LED is intercepted by the hand or the like, the output of the photodiodes is reduced by such a great amount which is never likely in the normal conditions and thus the wiper is brought into an undesirable operating condition.